Submarine cable insulation



Aug. ll, 1931. J. J. GILBERT 1,818,127

-SUBMARINE CABLE INSULATION Filed April 29. 192s 2 sheets-sneer 1 my\ yFresa/rr lbs. per s; /z

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' I SUBMARINE CABLE INSULATION Filed April 29, 192e 2 sheets-sneer 2 7/I /N vE/v ran J2 JOHN J @/mr 2 low temperature presentat great depths.

Patented Aug. 1.1,l 1931 UNITED STATES-PATENT OFFICE *Y JOHN JOSEPHGILBERT, OF PORT WASHINGTON, NEW "YORK, ASSIGNOB TO WESTERN ELECTRICCOMPANY, INCORPORATED, 0F NEW YORK, N. Y., A CORPORATION OF NEW YORKSUBM'ARINE CABLE INSULATION Application led April 29,4

This invention relates to submarine cables for telephony and telegraphyand particularly to cables for deep sea servicewhere the conductor issubjected to high pressure and 5 low temperature. l

One of the most serious problems in signaling by submarine cable is theattenuation of the signals. -While this is detrimental in low frequencytelegraphy, the effect is much o greater in high frequency telegraphyand the still higher frequencies of telephony for which present daycables are designed. More especially is this so when cables aresubjected to the extremely high pressure and An object of the presentinvention is to reduce the attenuation of signals transmitted oversubmarine cables.

A more specific object is the reduction of the leakance losses in suchcables. j One method of attaining these objects becomes evident from astudy of the factorsj upon which the attenuation constant of the 2K5-cable depends. This constant a may be calculated from; the formula weee,l

in which R is. the effective resistance, L the 5 inductance, Cthecapacity and G. the leakancenper nautical mile of cabl'e. The! eX- 4pression is considered as asingle quantity throughout this.specification because in the'testing of dielectric-materials it hasbecome .the general practice to consider thi? expression rather-thanthelleakance Gr alone. This is because the shape of the sample undertest will affect both the capacity and the leakance but in such a waythat the quantity .dielectric material. Considerable mathematicalcalculation isthereby avoided.

'1926. Serial No. 105,390.

It isobvious that forja cable having a given resistance, inductance. andcapacity,

the attenuation constant vvill have the smallest value when aninsulating material is employed which has a value of as small aspossible. It is also seen that the Aeffect'of lealrance upon theattenuation con loaded cable is largely dependent 'upon the leakancev ofthe sections of non-loaded conductor between the loading coils and mayThe leakance of a coil therefore be reduced by thesame means;

'which ivill reduce the leakance of al nonloaded cable.

In the formula for the attenuation con#A stant the quantity e is to bmultipliaiby the inductan L. yI

'ris' might therefore appear that for va so-called 1 non-loaded cablethe leakance .would have no effect. It must be remembered however,

that even a non-loaded? conductor has an appreciable inductance amountinhenry or more per nautical mile ue to the.

`magnetic fields set up in the sea water surrounding the cable. Y

Submarine cables when laid in deep water are ordinarily `subjected tovery great pressure due to the weight of the water above the cable, thedepth at which the cable is laid determining the amount of -thispressure.

Trans-oceanie cables are subjected to pressures of over v2500 pounds persquare inch for the greater part of their length and a considerableportion of they cable is'usuall'y subjected to pressures ranging from5000 to 10,000 pounds per square inch.' v

The temperature of the water at the ocean ybottom is usually in the neihborhood of 4 C. and-the temperature of t e cable and in- I sulation'istherefore the same.` l v- Although these extreme conditions ofpresexist,but little attention has been given to the effect upon the insulationmaterial other than to insure that the insulation was impervious towater under these conditions.

' The insulation used at the present time for submarine cables isusually gutta percha ap- Aplied in a layer about 120 mils or more thick,

the thickness depending upon such factors as the operating potential andthe insurance of mechanical integrity.

In the manufacture of submarine cables it is usual to apply a fluxmaterial, such as Chattertons compound to the conductor in order toinsure adhesion between the conductor and insulation. This flux isapplied in a very thin layer, about 1 mil'in thickness, before theinsulation is applied. In a copending application for A. R. Kemp, issuedas Patent No. 1,700,766 on February 5, 1929 is described a continuouslyloaded conductor in which bitumen was applied in a thick layer fillingthe interstices between the loading material and the conductor so as toact asa cushion substance to relieve strains set up in the loadingmaterial during the manufacture and the laying of the cable. It alsoserved the additional purpose of a flux for the gutta percha insulation.Up to the present time, however, there has been no use of bitumen as aflux between the conductor and the insulation of non-loaded or coilloaded cables.

The insulation materials applied to conductors heretofore have seldombeen chosen with reference to their characteristics under the actualoperating conditions of high pressure and low` temperature. It has,however, been known for some time, among cable manufacturers, that thecapacity and the leakance of insulation varies with changes in pressureand temperature and specifically that the leakance of gutta perchaincreases lwith an increase in pressure and with a decrease intemperature.

It has also been known that the leakance is, to a limited extent,proportional to the frequency of the impulses transmitted over thecable. F or'this reason the leakance of a Icable for high frequencytelegraph or telephone operation is considerablygreater than that of thesame cable for directcurrent .telegraph operation. Furthermore theattenuation constant is mostly affected by the characteristics of thelayers of insulation nearest the conductor, that is within a thicknessof from 20 to 30 mils from the surface of the conductor.

From experiments conducted in connection with the present invention itis now found that the quantity i for various substances depends upon thefrequency, temperature and pressure at which it is measured.

In accordance with this invention a submarine cable is contemplated forthe A4transmission of signals, the frequencies of which are those metwith in telephone work or high speed telegraphy, under conditions ofhigh pressure and low temperature present at great depths. Such a cablehas a material, combined with the usual insulation, which decreases theleakance of the cable, under conditions of high pressure. This materialis applied t0 the conductor and acts as a binder between the conductorand main insulation. Certain of the bitumens, and especially asphalt,have been found very satisfactory for this purpose. The inventionfurther contemplates a cable having one insulation, usually guttapercha, at moderate depths and a combination of gutta percha and amaterial such as asphalt for the eXtreme depths.

In the accompanying drawings Fig. 1 indicates the variation of forseveral materials when the pressure is varied and the frequency of themeasuring current 1s 100 cycles per second;y Fig. 2 is a 'set of curvesfor the same materials subjected to varying pressures when the frequencyof the measuring current is 1000 cycles per second; Fig. 3 shows a crosssection of a cable using asphalt as the flux or binder between theconductor and gutta percha; and Figfi'shows a cable with differentsections` constructed to meet the requirevments -at the respective oceandepths at phalt flux. For protection etc., a layer of jute 13 is usuallyapplied, then a layer of armor 14 and finally a second layer of jute 15.

The thickness of the layer of flux or binder is dependent upon thematerial employed but for asphalt this layer should be about l0 to 20mils. thick. Since the greater part of the leakance lossesoccur's withina thickness of about 30 mils. of insulation, it would seem that athicker flux would be of advantage but an increase in the thickness ofasphalt beyond about 2O mils presents considerable difficulty in themanufacture of the cable and is therefore not feasible.

lIt must be remembered that the capacity of an insulated conductor islargely determined by the dielectric constant of the malac terialclosest to the conductor and it is necessa n to insure vthat thearrangement de- Y -scrbed for reducing the value of does not increasethe lcapacity of the conductor and hence increase the attenuationconstant. U der sea bottom conditions the dielectric co stant of asphaltis lessv thanI that of`gutta percha so that a decrease in capacity ofthe cable accompanies the decrease in obtained by employing a layer ofasphalt between the conductor and the gutta percha. In Figs. 1 andQcurves A represent the variations in l due to varying ressures, at atemperature 25A of4 (land at requencies of 100 and 1000A cycles persecond, respectively, for a cable having a heavy layer; of asphalt fluxsurrounded by gutta percha as shown in Fig. 3.

for a cable with a heavyv layer of asphalt decreases with increasingpressure, whereas the value of for a cable with the gutta perchainsulation close to the conductor in general increases with thepressure. l.

' From the curves of Fig. 1 taken at a transmitting frequency of 100cycles per second it is apparent that with respect to ,the value and itseffect upon the attenuation constant v f the cable having a heavyasphalt layer has an advantage over the cable having the insulation ofordinary gutta percha close to the conductor when used on depths wherethe pressure is above 4500 pounds per square inch. At pressures around10,000 pounds per square'inch it has been found that 'the cable with alayer of asphalt in this respect is equal to that having the insulationof improved utta percha close to the conductor, the va ue of then' being14 in both cases.

The advantage of the asphalt flux is more pronounced at higherfrequencies. From the 'curves of Fig. 2 it is evident that the cablehaving a layer of asphalt is far superior at all pressures to thathaving the ordinary 'gutta percha insulation close to the conductor, andexcels at pressures above 2500 pounds per s uare inch'the cable havingthe improve gutta percha insulation close to the conductor.

The use of a heavy layer of ux, such as mentioned above, therefore, hasthe effect, not only of serving` as an effective binder, but also ofreducing the leakance of a cable at the higher Apressures to whichsubmarine cables generally aresubjected.

From a consideration of Figs. 1 and 2, it will be seen that someadvantages may be obtained` in both lowv frequency and high frequencycables by employing an insulation havinglow leakance under high pressurefor the deep sea portions of the cable and guttapercha or otherinsulation material having low'leakance at low pressures for theterminal or shallow water portions o f the cable. This could beaccomplished by employing a very thin layer of flux consisting of ashalt or similar material, 1 mil or so in thic (ness for shallow waterportions and a thicker layr of flux for the deep sea portions of the cae.

, Although bitumens and particularly asc phalt have been mentioned assuitable for this purpose, it is contemplated that any material whichhas low leakanceat high pressures and low temperatures may be employedto advantage.

The manner in which the insulation is applied to the conductor and itschemical composition is of little importance provided it has thedesirable properties described above and` is not in any way injurious tothe remainder of the cable. 1

Such an arrangement is shown in Fig. 4, in which the cable 20 is shownas having a section 21 placed at a comparatively great ocean depth and asection 22 ata smaller depth. The figure particularly shows detail crosssections 23 and 24 of the respective sections, the cross sectionsshowing a cable core similar to that shown in' Fig. 3 without theprotecting layers of jute and armoring. In the cross section 23corresponding to the greater depth the layer -31 of a low leakanceinsulating material, placed between the conductor 30 and the outerinsulation 32, is comparatively heavy. In the cross section 24corresponding to the smaller depths the layer 31 is comparatively thinand may merely serve as a binder. The selection of the insulatingmaterials and the dimensions of the insulating layers would of coursedepend upon the transmission frequency `and the depth at which the cableis to be located.

It is to be understood that this invention is applicable to the sectionsof coil loaded telegraph cable comprising a conductor and an insulatingmaterial closely surrounding said conductor, said insulating materialhaving a decreasing leakance with decreasing temperature.

3. A non-loaded submarine telephone or telegraph cable comprising aconductor and an insulating material closely surrounding said conductor,said insulating material having a decreasing leakance with increasingpressure and decreasing temperature.

4. A non-loaded section ,of a submarine tele-)hone or` telegraph cablecomprising a conductor, insulation for said conductor, and a secondinsulating means between said conductor and insulation and engaging saidconductor, said second insulating means having a lower leakance thanimproved gutta percha for frequencies of the order of 1000 cycles whensubjected tohigh pressure and low temperature. Y

5. A non-loaded section of a submarine telephone'or telegraph'cable fortransmitting frequencies of the order of 1000 cycles per second,comprising a. conductor, insulation `for for said conductor, and aninsulating flux betweenv said insulation and conductor and engaging saidconductor, said iux comprising a material having a leakance lower thanthat of improved gutta percha at pressures in eX- cess of 2500 poundsper square inch, and at temperatures in the neighborhood of 4 de? greescentigrade.

G. A non-loaded submarine telephone or telegraph cable comprising aconductor7 insulation for said conductor, and a layer of bitumen betweensaid conductor and insulation and engaging said conductor.

7. In a non-loaded submarine telephone or telegraph cable a conductor,insulation for said conductor comprising gutta percha, and means forreducing the leakance of said cable comprising a layer of bitumenbetween said conductor and said gutta percha insulation.

8. A submarine telephone and telegraph cable comprising a conductor andcomposite insulation therefor of bitumen and other insulating material,the amount of bitumen applied per unit length of the conductor beingsmaller on portions of the cable. near the terminals than on portionsmore remote from the terminals to compensate for the effect of thedifference in water pressures acting on said portions.

9. A submarine telephone or telegraph cable comprising a conductor, amain body of insulating material for said conductor and a layer of aninsulating material different from that in said main body between saidconductor` and insulation, the thickness of said layer being greater forcertain portions of the cable than for other portions of the cable tocompensate for the effect of the difference in waterpressures acting onsaid porf tions.

10. In a submarine telephone or telegraph cable, a conductor, aninsulation therefor,y

said insulation comprising an insulating material having a decreasingleakance characteristic with respect to pressure and another insulatingmaterial having an increasing leakance characteristic with respect topressure, the weight ratio between said materials being cable. Y

11. In a submarine telephone or telegraph cable, a conductor, aninsulation therefor, said insulation comprising an insulating materialhaving an increasing leakance characteristic with respect to temperatureand another insulating material having a decreasing leakancecharacteristic with respect to temperature, the"weight ratio betweensaid materials being different at different p0rtions of said cable.

12. In a submarine telephone or telegraph cable, a conductor andinsulation therefor, said insulation comprising an insulating materialhaving a decreasing leakance characteristic with respect to pressure andan increasing leakance characteristic with respect 1|() to temperature,and a second insulating material having an increasing leakancecharacteristic with respect to pressure and a decreasing leakancecharacteristic with respect to temperature, the weight ratio betweensaid materials being different at different portions of said cable..

13. A submarine cable for signaling fre-y quencies of 1,000 cycles persecondhaving a non-loaded deep sea section and a non-loaded a layer ofbitumen contiguous to said conductor.

In witness whereof, I hereunto subscribe my name this 27th day of April,A. D., 1926.

JOHN J. GILBERT.

different at different portions of saidv

